Method for removing chromate stain from galvanized metal

ABSTRACT

Method and composition for removing chromate stain from the surface of galvanized metal which has been treated with chromate for prevention of &#39;&#39;&#39;&#39;wet storage stain.&#39;&#39;&#39;&#39; The stained metal is treated with an aqueous solution of hydrogen peroxide, oxalic acid, sodium hypophosphite or formic acid, optionally containing phosphoric acid and boric acid. After treatment for stain removal, the treated metal retains its resistance to the appearance of &#39;&#39;&#39;&#39;wet storage stain.

United States Patent 11 1 Hill [ Oct. 2, 1973 1 1 METHOD FOR REMOVING CHROMATE STAIN FROM GALVANIZED METAL [75] Inventor: Uno T. Hill, Gary, Ind.

[73] Assignee: Inland Steel Company, Chicago, 111.

[22] Filed: Aug. 31, 1971 [21] Appl. No.: 176,721

[52] US. Cl 134/3, 134/2, 252/79.2, 252/100 [51] Int. Cl. C231 3/00, C231 11/00 [58] Field of Search 134/2, 3, 41; 252/79.1, 79.2, 94, 100; 148/615, 6.16, 6.2

[56] References Cited UNITED STATES PATENTS 10/1970 Thompson 148/62 X 11/1958 Prust 148/615 R 5/1961 Logan 148/615 R 3,506,499 4/1970 Okada 148/62 3,553,034 1/1971 Harvey.... 148/62 3,592,701 7/1971 Pekar 148/615 R 3,664,887 5/1972 Atkiss 148/65 R Primary Examiner-Morris O. Wolk Assistant Examiner-Sidney Marantz Attorney-Charles J. Merriam et a1.

[57] ABSTRACT 9 Claims, No Drawings METHOD FOR REMOVING CHROMATE STAIN FROM GALVANIZED METAL This invention relates to a method and composition for removing chromate stains from the surface of galvanized steel which has been inhibited against deterioration by treatment with a chromate.

Zinciferous metal surfaces, such as galvanized iron or steel surfaces, are subject to a form of deterioration known as wet storage stain" or dry rust, which is usually the result of a reaction between zinc, moisture, and oxygen, typically when the metal surfaces remain in contact with each other during storage. As a result of the reaction, the surfaces deteriorate and become covered with a white powdery substance which is not only unsightly but also seriously weakens the bond between the surface and any coating, such as paint, which may be applied thereto.

in order to prevent or inhibit the appearance of wet storage stain, it is conventional to treat galvanized metal surfaces with chromic acid or its derivatives, and particularly the salts thereof, such as water soluble alkaline earth and alkali metal salts, e.g., sodium and potassium dichromates. Typically, a sufficient quantity of the treating agent is applied to the galvanized surface to provide chromium in an amount between about 0.3 and L mg. per square foot of surface, a quantity which is sufficient to inhibit the formation of wet storage stain. When the amount of chromium applied to a zinciferous surface is below about 1 milligram per square foot no chromate stain is discernible on visual inspection. At times, however, this level of chromium is intentionally or accidentally exceeded for various reasons and a stained, unmarketable product results.

In accordance with the invention, l have discovered that a chromate stain on the surface of a zinciferous metal can be removed in that substantially affecting the resistance of the surface to wet storage stain, by treating the surface with an aqueous solution of hydrogen peroxide, oxalic acid, formic acid or sodium hypophosphite, or mixtures thereof, under conditions of time, temperature, concentration and pH which will be set forth below. I have further determined that treating solutions containing any one or more of these active ingredients can be further improved byincorporating therein a small concentration of phosphoric acid or a combination of phosphoric acid and boric acid, the presence of which additionally inhibits the subsequent appearance of wet storage stain after the chromate stain has been removed.

Treatment of a chromate stain on a zinciferous surface in accordance with the invention is accomplished simply by immersing the metal object in, or applying to its surface in any convenient fashion, such as by spraying or wiping, an aqueous solution containing at least one of the active ingredients contemplated for use in the invention, i.e., hydrogen peroxide, oxalic acid, formic acid, or sodium hypophosphite. Combinations of two or more of the active ingredients can also be used, with the concentration of each being adjusted proportionately.

The concentration of active ingredients in the treating bath depends on the identity of the active ingreditact time is less than this period, as in the case of an automatic high speed galvanized strip line, the concentration can be increased to the upper limit given in the table. Solutions more concentrated than those specified, however, may leave undesirable chemical deposits on the strip, or may result in corrosive attack on the metal and are therefore generally undesirable. Concentrations lower than the specified lower limit can be used if desired but require a concomitant increase in the treating time.

Concentration, Active ingredient k By Weight Hydrogen peroxide 3-6 Oxalic Acid 0.5-1.0 Sodium Hypophosphite 0.5-1.0 Formic Acid 2.5-5.0

The temperature of treatment can vary over a wide range, from a low of room temperature to an upper limit of about 200 F., the preferred temperature range being about l50-l80 F.

The treating solutions of the invention must have pH values within the range of about 1.5-6, and preferably about 2-3. In order to reduce the pH of solutions of hydrogen peroxide or sodium hypophosphite to values within the proper range, an appropriate quantity of a weak acid such as acetic acid, formic acid, or phosphoric acid can be added to the solutions. The use of phosphoric acid, in concentrations of about 0.2-0.4 percent by weight of the treating bath, is preferred for pH control. Although phosphoric acid may not be needed to adjust the pH of solutions of oxalic acid or formic acid, its use is advantageous for inhibiting the appearance of wet storage stain after treatment of the metal surface for removal of chromate stain in accordance with the invention. Accordingly, the use of phosphoric acid as an additive in all of the treating baths of the invention is a preferred embodiment of the invention.

It is also desirable to include in the treating baths in combination with phosphoric acid about 0.5-l .0 percent by weight of boric acid. The boric acid greatly increases the resistance of the treated zinciferous surfaces to appearance of wet storage stain.

EXAMPLE A The invention is illustrated by the treatment of galvanized steel bearing heavy chromate stains with the compositions given in Table l. The stains resulted from the application to the surfaces of the galvanized steel of chromate solutions in an amount sufficient to deposit about 5 milligrams or more of chromium per square foot of surface. in each case, coupons of the stained metal, about 4 X 6 in size, were immersed in each treating solution, which was maintained at a temperature of about l F., and allowed to remain therein for about 2 seconds. In each case the chromate stain disappeared within the period of treatment, producing a bright finish. The metal coupons, wet with the treating solution, were then passed through a ringer roll to remove excess liquid and allowed to dry in air. The pH of each treating solution was about 2, either because of the inherent acidity of the active ingredient, or the addition of a sufficient quantity of phosphoric acid to achieve the pH value.

The treating solutions which were tested and which in each case were effective to remove the chromate stain within a period of less than 2 seconds were aqueous solutions containing the indicated additives dissolved therein.

TABLE I Concentration,

Solution Additives I: By Weight (1) hydrogen peroxide 3.0

phosphoric acid 0.2

(2) oxalic acid 0.5 (3) sodium hypophosphite 0.5

phosphoric acid 0.2

(4) formic acid 2.5

EXAMPLE B Although treatment with the compositions of the invention effectively removes chromate stains from galvanized metal, the resistance to wet storage stain of the treated surfaces remains at a high level. This effect was demonstrated by a test in which the resistance to wet storage stain of the coupons treated with the compositions given in Table l above was detennined. In the test, which has been found to correlate well with results in the field, the treated coupons of metal were wetted with water, stacked with adjacent surfaces in contact and stored in a humidity cabinet maintained at a temperature of 50 C. and having an atmosphere of air saturated with water vapor. As controls, there were used similar coupons of virgin galvanized steel which had been given no surface treatment of any kind. Periodic examination of the test coupons was made to determine the appearance of wet storage stain.

In addition to the coupons which were treated with the compositions given in Table 1, other chromatestained galvanized steel samples were treated with the compositions given in Table II for removal of the chromate stain, following the procedure described in Example A.

TABLE ll Concentration. Solution Additives By Weight (5) hydrogen peroxide 3.0 phosphoric acid 0.2 boric acid 0.5 (6) oxalic acid 0.5 phosphoric acid 0.2 (7) oxalic acid 0.5 phosphoric acid 0.2 boric acid 0.5 (8) sodium hypophosphite 0.5 phosphoric acid 0.2 boric acid 0.5 (9) formic acid 2.5 phosphoric acid 0.2 (10) fonnic acid 2.5 phosphoric acid 0.2 boric acid 0.5

In each case the solutions of Table ll removed the chromate stain within 2 seconds, as in Example A.

The results of the test for wet storage stain resistance showed that whereas the untreated control metal coupons invariably exhibited wet storage stain within 2-3 hours, the coupons treated with the solutions of Tables I and ll resisted the appearance of wet storage stain for periods approaching or exceeding 40 hours, indicating good resistance to wet storage stain in actual use. Although the resistance to wet storage stain of coupons treated for chromate stain removal with oxalic acid and formic acid alone was much superior to that of the untreated coupons, the addition of phosphoric acid to the treating baths, as indicated in the solutions of Table ll, markedly improved the resistance to wet storage stain. The addition of boric acid further improved the resistance to such corrosion of coupons treated with those baths containing added phosphoric acid.

In addition to removing unsightly chromate stains from zinciferous metals while maintaining a high resis tance to wet storage stain, the method of the invention also helps to condition the surfaces for painting, by improving the adhesion of paint applied to such treated surfaces. Thus in many cases it may be possible to omit more expensive pretreatments intended to prepare galvanized steel surfaces for the application of paint.

The forms of invention herein shown and described are to be considered only as illustrative. It will be obvious to those skilled in the art that numerous modifications may be made therein without departure from the spirit of the invention or the scope of the appended claims.

I claim:

I. A method of removing visible chromate stain from the surface of zinciferous metals which comprises ap plying to said stained surface an aqueous solution containing dissolved therein an effective concentration of a compound selected from the group consisting of hydrogen peroxide, oxalic acid, sodium hypophosphite, formic acid, and mixtures thereof, and permitting said solution to stay in contact with said stained surface until said stain is removed, said solution having a pH of about l.5-6.

2. The method of claim 1 wherein the concentration of said dissolved compound in said solution is within the following ranges:

Concentration, 1:

Compound By Weight (a) hydrogen peroxide 3-6 (b) oxalic acid 0.5-1.0 (c) sodium hypophosphite 0.5-1.0 (d) formic acid 2.5-5.0

3. The method of claim 2 in which said solution also contains dissolved therein about 0.2-.4 percent by weight of phosphoric acid.

4. The method of claim 2 wherein said solution has a pH of about 2-3 and is maintained at a temperature between room temperature and about 200 F.

5. The method of claim 4 wherein said solution has a temperature of about l50-l80 F.

6. The method of claim 2 in which said solution also contains dissolved therein about 0.2-0.4 percent by weight of phosphoric acid and about 0.5-1.0 percent by weight of boric acid.

7. The method of claim 6 wherein said solution has a pH of about 2-3 and a temperature of about l50l F.

8. The method of claim 7 wherein said dissolved compound is oxalic acid.

9. The method of claim 7 wherein said dissolved compound is hydrogen peroxide.

# t t i i 

2. The method of claim 1 wherein the concentration of said dissolved compound in said solution is within the following ranges: Concentration, % Compound By Weight (a) hydrogen peroxide 3-6 (b) oxalic acid 0.5-1.0 (c) sodium hypophosphite 0.5-1.0 (d) formic acid 2.5-5.0
 3. The method of claim 2 in which said solution also contains dissolved therein about 0.2-.4 percent by weight of phosphoric acid.
 4. The method of claim 2 wherein said solution has a pH of about 2-3 and is maintained at a temperature between room temperature and about 200* F.
 5. The method of claim 4 wherein said solution has a temperature of about 150*-180* F.
 6. The method of claim 2 in which said solution also contains dissolved therein about 0.2-0.4 percent by weight of phosphoric acid and about 0.5-1.0 percent by weight of boric acid.
 7. The method of claim 6 wherein said solution has a pH of about 2-3 and a temperature of about 150*-180* F.
 8. The method of claim 7 wherein said dissolved compound is oxalic acid.
 9. The method of claim 7 wherein said dissolved compound is hydrogen peroxide. 